There is no guarantee the £55 billion Northern Rock has received from taxpayers will ever be repaid, the bank's new chief executive warned yesterday.

Ron Sandler, the man called in by the Government to save Northern Rock, sounded the warning after saying it would take "some years" for Northern Rock to pay back the money.

Mr Sandler, a respected troubleshooter who rescued Lloyd's of London from the brink of collapse, warned: "All of the possible solutions carry with them an element of risk."

He said suggestions that the bank might repay its £25 billion loan from the Treasury within months were unrealistic - and the bank would also continue to need Treasury guarantees protecting the savings of investors, currently estimated to be costing taxpayers another £30 billion.

Mr Sandler warned: "I do think it's clearly unrealistic to be talking about months - we are clearly talking about a period of some years."

It means the fate of the bank is still likely to be undecided during the next General Election, which will be in 2010 at the latest.

The task of making nationalisation work got under way yesterday, as Gordon Brown, the Prime Minister, and Alistair Darling, the Chancellor, faced a turbulent day at Westminster.

Mr Darling told the Commons the bank would be taken into temporary public owner-ship until it could be returned to the private sector "when market conditions made that possible".

He said he had put the interests of taxpayers first when he announced over the weekend that Northern Rock would be nationalised.

Mr Darling was also under pressure from some MPs representing constituencies in the North-east to ensure restricted job cuts among the bank's 6,000-strong workforce.

But the Conservatives called for Mr Darling to be sacked, claiming he had lost all credibility as Chancellor.

The Tories warned they would oppose an emergency Bill allowing the Government to take control of the bank when it is considered by the Commons today, although the Government will have no difficulty winning a majority thanks to the support of its own MPs.

The first bank nationalisation since 1984 will leave the Treasury responsible for £113 billion of Northern Rock assets and 6,500 employees.

A consortium led by Richard Branson's Virgin Group had put forward alternative proposals for a private takeover, but Mr Darling yesterday said these would not have represented good value for the taxpayer.

He told the Commons: "Under public ownership, the taxpayer will secure the entire proceeds from the future sale of the business in return for bearing the risks in this period of market uncertainty."

He added: "In deciding which was the best option for the taxpayer, it was clear that a temporary period of public ownership was the better option."

David Cameron accused the Government of leaving every taxpayer exposed to liabilities of £3,500 through "incompetence".